The basal forebrain cholinergic system consists of the nucleus basalis of Meynert, the medial septum and the diagonal band of Broca. The first of these nuclei provides the major source of cholinergic input to the cortex and amygdala, whereas the others provide the cholinergic input to the hippocampus. Dysfunction of this cholinergic system has been proposed as one explanation for the memory impairments observed in Alzheimer's disease. To assess the contributions of these cholinergic areas to recognition memory, we compared the effects of neurotoxic lesions of nucleus basalis alone, medial septum plus diagonal band, or all three regions combined. The results suggest that combined damage to all three is necessary to produce significant impairment in recognition memory in monkeys, perhaps because only such damage causes cholinergic denervation of both the amygdala and the hippocampus. In a separate study in normal monkeys, we found that the cholinergic antagonist scopolamine, in a dose range that impairs recognition memory, has no effect on habit formation, as measured by an object discrimination task involving 24-hour intertrial intervals.